Modern RF power circuits, for instance amplifier circuits for the gigahertz range, in many cases have a circuit structure characterized by a division of highly loaded active components into functional blocks of identical type. Specifically in the case of transistors this is also referred to as a cell structure if, for instance, large emitter/gate areas of RF power transistors are divided into transistor cells which are typically lined up one after another.
In the case of this block or cell structure that has been known for a long time, according to recent insights functional irregularities occur in extreme RF applications and, under certain circumstances, restrict the possibilities for using the corresponding RF circuit arrangements.
Thus, simulation calculations and measurements by means of the method of secondary photon spectroscopy have revealed that the collector current profile in different cells of a transistor connected symmetrically via a wide metal interconnect differs considerably in its temporal dependence relative to one another and/or with respect to the temporal profile of the control voltage.
FIG. 1 schematically shows the wiring structure of such a known transistor arrangement, and FIG. 2 shows a graphical representation of the time dependencies of the collector current for the individual transistor blocks. Details of the graphical representation are not of importance here; the illustration reveals in any case considerable deviations of the temporal dependence from an ideal sinusoidal curve and also between the individual time profiles, which indicates a nonuniform capacity utilization of the overall RF transistor.
Besides the approach of minimizing the resistive component of the lead (as pursued with the wide connection block in accordance with FIG. 1), also known is the approach of constituting the resistive component of the connection wiring as symmetrically as possible by geometrically symmetrical design of the lead portions. This approach is illustrated schematically in FIG. 3, and FIG. 4 shows a time profile of the collector current measured for this wiring structure, under otherwise identical conditions to FIG. 2. Here, too, the details are not of interest—what is striking, however, is the severe deviation from an ideally sinusoidal time profile, which indicates inhomogeneous capacity utilization of the RF power transistor connected in this way.